Modular multilevel converters have become popular because of their high efficiencies, their modularity and scalability, as well as for their ability to produce voltage waveforms with low harmonic content which effectively reduce the need for large AC filters.
Several modular multilevel converter topologies exist, of which the M2LC (also called MMLC and MMC) is one of the most popular, in particular in high-voltage direct current (HVDC) applications but also in motor drives etc.
WO 2010/124706 and WO 2011/060823 discloses the traditional half bridge and H-bridge converter cells for M2LC with batteries connected through DC-DC converter interfaces. The DC-DC converters can be used to filter alternating current (AC) components out from the battery current, and to boost the battery voltage to match the direct current (DC) link capacitor voltage.
The M2LC converter may be used as the converter interface for Battery Energy Storage Systems (BESS). The DC energy storage device(s) (Li-ion, Pb-Acid or NaS batteries, supercapacitors or similar) can be connected either to the high voltage common DC link of the M2LC or they can be distributed within the arms of the converter as proposed in WO 2010/124706 and WO 2011/060823.
The very high voltage of the common DC link of the M2LC is a disadvantage in the BESS application, since it is not trivial to build a battery system with such high voltage. The high voltage adds cost for insulation, as well as for fault handling and circuit breakers. The battery system can be equipped with a distributed DC breaker based on insulated-gate bipolar transistors (IGBTs). It adds a significant cost to the system, and produces losses. Low voltage energy storage devices that are distributed within the arms of the modular multilevel converter avoid the challenges related to a high voltage DC system and have the potential to reduce the costs for protection. Distributed energy storage within the arms also provides a higher level of redundancy.
Electrochemical batteries and supercapacitors will vary their voltage during a charge-discharge cycle. The discharge profile of a battery depends mainly on battery chemistry, but is also influenced of battery temperature, charge-discharge rate, aging of battery etc. Any kind BESS converter will need to handle the variable DC voltage of the battery(-ies), and that comes to an expense of de-rating (voltage-wise). Thus, if batteries are interfaced directly in parallel with the cell capacitors, the cell capacitors will have to vary their voltage accordingly, resulting in a significant de-rating of the whole converter.
The DC link capacitor voltage in an M2LC or chain-link converter will be exposed to fundamental and 2nd harmonic frequency components. Distributed energy storage devices connected directly to the cells' DC link capacitors will therefore share the harmonic currents. This will lead to increased losses in the storage device, higher operating temperatures and decreased lifetime.
DC-DC converters such can be used to avoid de-rating of the main M2LC converter and to filter harmonics from the battery current.
EP 1 920 526 discloses a converter circuit having at least one phase module comprising an upper and a lower converter valve, with each converter valve comprising at least one two-terminal subsystem. Each two-terminal subsystem comprises four turn-off capable semiconductor switches, four diodes, two unipolar storage capacitors and an electronic circuit. A diode is electrically connected in antiparallel with each turn-off capable semiconductor switch. These four turn-off capable semiconductor switches are electrically connected in series. The two unipolar storage capacitors are electrically connected in series, with this series connection being electrically connected in parallel with the series connection of the turn-off capable semiconductor switches. Each junction between two turn-off capable semiconductor switches forms a reconnecting terminal of the two-terminal subsystem. A junction between the two storage capacitors that are electrically connected in series is electrically connected to a reference potential terminal of the electronic circuit. However, EP 1 920 526 is not related to distributed energy storage within the converter.